The direct reduction of iron ore and pelletized iron oxide by a high temperature reducing gas prepared from natural gas and recycled spent reducing gas has become a significant commercial route toward the production of steel. The high temperature reducing gas prepared from natural gas has a high concentration of reducing constituents, carbon monoxide and hydrogen, as compared to the oxidizing constituents, steam and carbon dioxide. The ratio of these constituents (reducing to oxidizing) is called reducing ratio. As natural gas declines in availability and increases in cost, alternate routes are needed to produce a high temperature reducing as from coal and other fuels such as oil. The growing overdemand for oil, resulting in excessive prices, makes coal the fuel of choice for future processes to produce direct reduced iron from hot reducing gas.
Three basic types of coal gasification are conventionally available on a commercial scale, the entrained bed process, the fixed bed process, and the fluidized bed process.
The entrained bed process produces a reducing gas at about 1500.degree. C. and atmospheric pressure by the concurrent reaction of oxygen and steam with entrained coal dust. The gas from the process has a reducing ratio of about 5 but must be cooled before the gas can be compressed to the approximately two atmospheres of pressure required for direct reduction and also to allow removal of carbon dioxide, water and gaseous sulfur compounds before heating and use in direct reduction. Such cooling, cleaning and subsequent reheating is too costly in both equipment investment and energy loss to make the process highly attractive.
The fixed bed coal gasification process features a descending bed of coal moving countercurrent to an ascending gasification gas stream. The gasification gas is initially oxygen and steam at the bottom grate of the gasifier. As the gasification gases pass up through the descending bed of coal several zones are encountered. The first zone at the bottom discharges ash almost entirely free of carbon from the gasifier. In the next zone gasification gases oxidize the char from the coal to form hydrogen, carbon monoxide and carbon dioxide. In the next higher zone devolatilization of the organic content of the coal takes place as well as some gasification reactions. The gasification gas, now being rich in hydrogen and CO as well as methane and higher hydrocarbons such as naphthas and tars, passes to the next higher level in the bed where dewatering and preheating of the coal bed takes place. The discharge gases therefore contain large quantities of vaporized water, CO.sub.2, CO, hydrogen, some methane, naphthas and tars. Before such a gas can be used, the sulfur compounds, steam, CO.sub.2, naphthas and tars must be removed. This is best accomplished through the use of low temperature or ambient temperature removal systems. The cooling equipment, the cleaning and subsequent reheating of the gas is expensive, both in equipment investment and energy loss.
As is known to those skilled in the art, the only fully commercial fluid bed process presently available for the gasification of coal operates at atmospheric pressure and produces a gas which contains high concentrations of oxidizing constituents, steam and carbon dioxide. Before this gas can be used in direct reduction it must be cleaned of dust, compressed and then cleaned of carbon dioxide and sulfur compounds. Not only are the cooling and reheating steps expensive, but the gasification process itself is unable to utilize a large fraction of the carbon fed to the gasifier. Char is produced as a by-product and must be used in other processes. Newly developing fluid bed processes carry out the gasification under pressure and one process has a hot zone within the fluid bed where ash is agglomerated and allowed to fall from the bed. Char and ash removed from the discharge gas by a cyclone system is returned to the hot zone to obtain high utilization of the char and to remove the ash in agglomerated form. The purpose of the system is to obtain a high conversion of coal to gas by minimizing the withdrawal of char from the system. Because of the cyclone return system, the process offers the ability to accept fines in the coal feed. Even under the best of conditions however, the gas quality has not exceeded a ratio of 2, primarily because of the need to feed excess steam into the gasifier to cool the char in the fluid bed to prevent agglomeration. As a consequence the gas cannot be used without cooling, purification and reheating. These processes are undesirable for investment and energy reasons.
Note that reducing gas quality is commonly expressed as the ratio of reductants (CO+H.sub.2) to oxidants (CO.sub.2 +H.sub.2 O) in the gas mixture. In order to take full advantage of the chemical efficiency of a counterflow shaft direct reduction furnace, the qualify of the hot reducing gas introduced to the furnace should be at least about 8.